After adjusting for multiple confounders, the results remained similar for frozen versus fresh embryo transfer (odds ratio 0.90, 95% CI 0.66-1.47 for ongoing pregnancy, OR 1.01, 95% CI 0.73-1.40 for live deliveries), they reported at the European Society of Human Reproduction and Embryology annual conference.

Adjustments included all variables related to the cycle outcome, such as patient's age, serum E2, endometrial thickness, oocyte yield, fertilization rate, PGS, number of embryos transferred, and embryo stage at transfer.

"Most studies so far have included patients who are high responders to ovarian stimulation with high steroid levels at the end of the follicular phase," Bosch said. "These high levels of hormone have been shown to impair the receptivity of the uterus to embryo implantation, and this may explain why these studies have shown a benefit in outcome."

Because this study included patients who were "normo-responders" to ovarian stimulation, he told MedPage Today via e-mail that the results of his own study were not surprising.

"When we analyzed the data we already had the feedback of our embryologists that were telling us that this strategy was not improving our success rate," Bosch said.

He added in a statement, "These findings do not support a change in IVF practice moving to a freeze-all strategy in normo-responders in IVF," though included the caveat that his team did not include "high responders" -- women with a heightened response rate to ovarian stimulation -- and for them, frozen transfer is still recommended.

In this single-center retrospective cohort study, Celeda and colleagues examined data from the Instituto Valenciano de Infertilidad in Spain (n=882) from January to December 2013. The sample included 41.3% fresh embryo transfers (n=364) and 58.7% frozen embryo transfers (n=518), with the choice based on the preference of their doctors. Demographic characteristics included patients ages 20 to 44, undergoing first IVF/ICSI cycle, with a normal oocyte yield (4 to 20). No differences in demographic characteristics or ovarian stimulation parameters were observed between the two groups.

Authors note two potential limitations to the study -- its retrospective nature and the fact that the doctor, not the patient, decided on the frozen versus fresh transfer method.

Because fresh embryo transfer is more cost efficient with a shorter treatment time, Bosch said that he recommended further study and cited research already being conducted on this issue.

"There are at least two large randomized trials currently ongoing that will address this issue with better accuracy than ours," he said. "We need to wait for these studies to be completed in order to get a final conclusion."

Birth rates for women over age 38 did not substantially increase when using embryos from a frozen cycle, suggesting the benefit of cryopreservation may be limited as a woman's age increases.

Marta Devesa, of Hospital Universitario Quirón-Dexeus in Barcelona, Spain, and colleagues found that live birth rate (LBR) from frozen cycles decreased as age increased (P<0.001).

They noted that cryopreservation only increased LBR when compared with cycles that did not cryopreserve, but saw "limited" extra benefit in these frozen cycles.

Devesa and colleagues stratified women into four groups based on age. A predictive model adjusting for age and oocyte yield resulted in an estimated "cumulative live birth rate" (CLBR):

G1 (38-39 years): 23.6%

G2 (40-41 years): 15.6%

G3 (42-43 years): 6.6%

G4 (≥44 years): 1.3%

CLBR was also associated with an increasing number of oocytes, but the authors wrote that age appeared to play a significant role. While 12 oocytes yielded a CLBR of 36% in group G1, the CLBR was only 2% in group G4. But they note that CLBR in group G4 was unable to reach 3%, regardless of the number of oocytes.

Not surprisingly, LBR from fresh cycles only declined with increasing age:

G1: 20.3%

G2: 13.2%

G3: 6.1%

G4: 1.2%

In a statement, Devesa said that older women using donor eggs have had success in carrying pregnancies to term, suggesting it is not the age of the patient, but the age and "quality" of the eggs that plays a more significant role. She cited "U.S. data" showing a 60% success rate in embryo transfer for all ages of women, even those in their late 40s, but noted that after age 38, a greater portion of women use donor eggs (around 20% at age 42 to 60% at age 45).

"Embryo aneuploidy rates as high as 85% have been described in women older than 42," Devesa explained.

This retrospective study was conducted using data from a university-affiliated clinic from 2000 to 2012. The sample of 4,195 women underwent 5,841 IVF cycles, and comprised 2,119 cycles in group G1, 1,883 in G2, 1,159 in G3, and 680 in G4.

Limitations to the study include its retrospective nature.

Devesa called this the largest study ever to analyze live birth rates from IVF in women over 38, who represent a significant portion of IVF patients.

"Indeed, women of 44 or older should be fully informed about their real chances of a live birth and counseled in favour of oocyte donation," she said. "This is what probably explains the higher rates of oocyte donation in the U.S. than in Europe."

Devesa concluded that women over 44 should be advised against using their own eggs, while those younger than 44 should know that age and number of eggs are the best predictors of a successful outcome.

Celada and colleagues were funded by Instituo Valenciano de Infertilidad (IVI), Valencia, Spain.

The study for Devesa and colleagues was performed under the auspices of Cátedra d'Investigació en Obstetrica i Ginecologia of the Department of Obstetrics and Gynecology, Hospital Universitario Quirón Dexeus, Universitat Autónoma de Barcelona.

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